Vertical Hall sensors have the advantage that they can be manufactured using conventional semiconductor technology (e.g. CMOS). They have, however, the disadvantage that they suffer from a relatively high offset voltage compared to horizontal Hall sensors. The offset voltage is the voltage measured when no magnetic field is present.
The measurement error due to offset voltage can be decreased by several techniques, such as for instance the spinning current technique. In this technique, the supply and readout contacts are cyclically interchanged. In one phase the offset is then added to the magnetic signal with positive sign and in the other phase with negative sign. By averaging both phases, the offset can typically be reduced more than 100 times.
A Hall sensor may be represented as a distributed resistive Wheatstone bridge. In order for the spinning current technique to be effective it is important that electrical symmetry exists within the vertical Hall sensor (i.e. that all resistors in the resistive Wheatstone bridge have about the same value). A vertical Hall element, however, does not provide this electrical symmetry by default since all contacts are on one side and since the active area (e.g. an n-well) only has a limited depth. Therefore, in practice, rather than a factor 100, the offset is only reduced by a factor 5-10 after applying the spinning current technique. Moreover, the residual offset is temperature dependent and the dependency is non-linear. This requires a time consuming calibration procedure.
EP1540748 discloses a magnetic field sensor with a Hall element that has two inner and two outer contacts arranged along a straight line. The contacts are arranged on the surface of a well of a first conductivity type embedded in a substrate of a second conductivity type. A first of the two outer contacts and a first of the two inner contacts which are not adjacent contacts act as supply contacts to which a current can be applied, and the second of the two outer contacts and the second of the two inner contacts act as sense contacts for tapping a Hall voltage. A first resistance is provided between the first contact and the second contact, a second resistance is provided between the second and the third contact, a third resistance is provided between the third and the fourth contact, and a fourth resistance is provided between the first and the fourth contact. The resistance values of the first, second and third resistances are substantially equal. An additional resistance is furthermore provided, coupled in parallel to the fourth resistance, and having a resistance value selected such that the parallel resistance value of the fourth and the additional resistance is substantially equal to the resistance value of any of the first, second or third resistances. This way, the equivalent Wheatstone resistor bridge is balanced. With proper dimensioning of the additional resistor this works and can reduce to zero the residual offset after spinning. However, this works only at one fixed operation condition concerning biasing voltage and temperature. If any of these two parameters varies, the matching is disturbed by additional effects arising from the non-perfect symmetry between inside and outside contacts, leading to a strong increase of the residual offset.
The minimum magnetic flux density which can be accurately measured depends on the offset error of the vertical Hall sensor: the smaller the offset error, the more accurately a magnetic flux density can be measured. Therefore, in view of the difficulty in reducing offset errors, there is still room for improvement in vertical Hall sensors.